1. Field of the Invention
This invention relates to electrochemical cells.
2. Description of the Prior Art
One type of the electrochemical cell is a fuel cell. A typical fuel cell is comprised of a matrix material for holding electrolyte and an electrode disposed on each side of the matrix and in contact therewith. Reactant gases are fed to the nonelectrolyte facing side of each electrode. In a stack of fuel cells gas impervious separator plates are disposed between adjacent cells. The cells convert the reactants, such as hydrogen and air, (i.e., oxygen) into DC electrical power in a manner well known in the art. The electrochemical reaction produces, as a by-product, waste heat which must be removed in a controlled manner to maintain the cells at the desired operating temperature. For most efficient operation it is desirable to maintain all cells at a uniform temperature and at a maximum level consistent with material compatibility characteristics.
In the most common type of fuel cell stack the gas separator plate is relatively thick and includes grooves on each side thereof which carry the reactants across electrodes disposed on opposite sides of the separator. Representative of this type of cell construction are commonly owned U.S. Pat. Nos. 3,990,913; 3,964,930; and 4,157,327. Another type of cell construction is shown in commonly owned U.S. Pat. No. 4,115,627. In that patent the gas separator is a thin flat plate. The electrode is made thicker and the channels for carrying reactant gas are formed in the non-matrix facing side of the electrode. Yet another construction is shown in commonly owned U.S. Pat. No. 4,129,685. In that fuel cell structure there are no reactant gas channels. The electrodes, or more precisely the electrode substrate, is made quite thick and includes enough pores which are sufficiently large to permit a substantially free flow of the reactant gas therethrough both perpendicular to and parallel to the substrate. In one embodiment a smaller pore "reservoir layer" is disposed between this highly porous electrode layer and the gas separator and serves the function of storing excess electrolyte volume during cell operation.
Until now cell cooling has been accomplished by flowing a cooling fluid through channels or passageways in the gas separator plate. Examples of cooling by this technique are shown in aforementioned commonly owned U.S. Pat. Nos. 3,990,913; 3,964,930; 4,157,327. In these designs coolant carrying tubes pass through some of the separator plates parallel to the surfaces of the electrodes. For example, every third or fourth separator plate in a stack of cells may carry this coolant flow. Note that in all of these designs the separator plate is also required to include grooves on each side thereof for carrying the reactant gases to the electrodes on either side thereof. It is apparent from the foregoing that separator plates which are required to carry coolant fluid must be made considerably thicker than separator plates which are not required to carry coolant fluid. Generally these plates are high density, nonporous graphite made by molding and graphitizing a mixture of resinous material and graphite powder. The thicker the plate the more difficult it is to make the plate defect free and to close tolerances due to the high temperature graphitizing steps. Also, thicker plates cannot be made as dense as ultimately desirable and they, therefore, adsorb too much electrolyte. This reduces cell life.
In U.S. Pat. No. 3,990,913 the separator plate carrying the coolant tubes is made in two halves with semicircular grooves cut into each half, the halves mating together to form the circular channels within which the coolant tubes are alternately disposed. Once again, a disadvantage of this design is the difficulty in forming thick, gas impervious, graphite plates to close tolerances such that the plate halves mate properly. This is particularly true if it is desired to mold these pieces. A thermal caulk or grease must surround the coolant tubes to fill in any gaps therearound in order that adequate thermal conductivity between the cells and the coolant fluid is obtained. Note, in FIG. 2 thereof, that the gas separator 18a with cooler tube passageways requires about twice the thickness of a gas separator such as 18 which does not include the cooler tube passageway.
In U.S. Pat. No. 3,964,930 the overall thickness of the separator plate carrying the cooler tubes has been reduced by forming the cooler tube channels in the surface of the plate adjacent one of the electrodes. The cooler tubes, in this configuration, are more exposed to the corrosive environment of the electrolyte; but this is compensated for by a coating of electrolyte resistant perfluoroalkoxy or polytetrafluoroethylene on the external surface of the tubes. One disadvantage of this construction is that the tube carrying passageways must necessarily take the place of reactant gas carrying channels; this reduces the uniformity of reactant gas flow to the electrodes and requires an increase in the thickness of the gas separator plate so that the remaining gas channels can be made larger so as to accommodate the additional reactant gas normally carried by the missing channels. While the overall size of this gas separator plate is less than the overall size of the gas separator plate shown in U.S. Pat. No. 3,990,913, it is still thicker than plates which do not carry coolant.